1. Field of the Invention
The present invention relates generally to amplifier circuits, and specifically, to high performance amplifier circuits.
2. Background Information
There are many circuit applications that require analog outputs that are minimally degraded by noise on the power supply. Examples of these applications include high quality signal processing, signal conditioning, laptop/notebook computers, cell phones and portable headphone speaker drivers. These applications require analog amplifiers that exist on the same printed circuit board (PCB) as other high speed/high power circuitry and, often, very noisy switch-mode power supplies. These high speed/high power circuits tend to corrupt the power supply voltage, resulting in poor circuit performance. A highly regulated power supply may reduce the effect of noise, but the headroom voltage which is demanded by the regulator is lost for the amplifier and therefore, the output only has a rail-to-rail swing less the headroom loss in the regulator.
Traditionally, an amplifier can either have a good drive capability, i.e., a full rail-to-rail swing but a poor supply rejection or a good supply rejection but poor drive capability. Therefore, there is a need to have an amplifier circuit that has good supply rejection and full rail-to-rail output range without incurring hardware complexity.
The present invention is an apparatus and method to provide high supply rejection and rail-to-rail output swing in an amplifier circuit. The amplifier circuit includes a first stage that generates a first output. The first stage is powered by one power supply. The amplifier also includes a second stage that is coupled to the output of the first stage to generate an output, which second stage is powered by another supply. The two supplies are different in a regulation mode. In a preferred embodiment, the supply powering the first stage is derived from the other supply through a regulator providing high supply rejection, but not affecting the rail-to-rail swing of the second stage. This minimizes the power supply noise amplified by the first stage, with the voltage gain of the second stage regaining the rail-to-rail swing lost by the first stage due to the regulator supplying very low noise power to the first stage.